Added check for metadata inside createRotatedChannels() so it
doesn't try to do the rotation if we don't have the necessary
horizontal channel metadata (e.g., azimuths).
Modified channel loops to use all channels in the metadata (for AvailabilityMetric
and GapMetric) -or- to use 3 channels for -both- primary ("00") and secondary ("10")
sensors (for powerband metrics).  Moved the actual computation for each metric
into a metric-defined method (=computeMetric()).